The present invention is directed to temporarily protecting glass sheets, and, in particular, to temporarily protecting the surfaces of glass sheets used in producing liquid crystal displays (LCDs). The invention is useful, for example, in protecting such sheets from ambient contaminants and for preventing glass chip adhesions to the sheets during cutting or grinding. In addition, the invention can protect the sheets from scratching.
Many uses of glass, including LCD glass, require a very clean glass surface that is substantially free of dust and organic contaminants. When exposed to the environment, glass can quickly become contaminated with organic contaminants, with contamination being observed within a few minutes.
Current procedures used to cut and grind glass surfaces and edges often generate small glass chips (e.g., chips having a size greater than 1 micron and less than about 100 microns). Some of these particles irreversibly adhere to the clean glass surface, rendering the glass useless for many applications. This is particularly a serious problem in the case of LCD glass surfaces.
LCD glass can be made by a fusion draw process, which yields flat, smooth glass surfaces which can be cut or ground to the desired size. Some of the glass chips generated from the cutting process originate from the surface of the glass. When the flat surface of these chips comes into contact with the surface of the glass plate, there can be a large contact area between the chips and the glass surface which promotes strong adhesion. If a water film condenses between these two surfaces, permanent chemical bonding may occur, in which case the adhesion of the glass chips to the surface becomes irreversible.
One known method of protecting glass sheets, specifically, sheets of LCD glass, is to apply a polymer film on both major surfaces of the glass to protect the glass during the scoring, breaking and beveling processes. In a typical method, one major surface has a polymer film attached with an adhesive, and the other major surface has a film attached by static charge. The first film is removed after the edge finishing (cutting or grinding) of the sheet is completed, while the second is removed prior to the finishing process. Although the adhesive-backed film protects the surface from scratching by the handling equipment, it causes other problems. For example, the polymer may entrap glass chips produced during the finishing process, leading to a build up of glass chips and scratching of the glass surface, particularly near the edges of the surface. Another problem with this film is that it may leave an adhesive residue on the glass surface. A further problem with the film approach is glass breakage during peeling of the film from the glass surface, especially for large and/or thin glass sheets. There is a need, therefore, for a method of protecting a glass surface from chip adhesions that does not leave residual coating on the glass surface, and for a method of temporarily protecting glass surfaces, whereby a glass article with a clean, coating-free surface can be readily obtained for further use.
A critical aspect of any coating used to temporarily protect LCD glass is removability. Manufacturers of liquid crystal displays use LCD glass as the starting point for complex manufacturing processes which typically involve forming semiconductor devices, e.g., thin film transistors, on the glass substrate. To not adversely affect such processes, any coating used to protect LCD glass must be readily removable prior to the beginning of the LCD production process.
In view of the foregoing, there has been a need in the art for a method for protecting glass sheets, specifically, sheets of LCD glass, which has the following characteristics:
(1) the method preferably should be one that can be readily incorporated in the overall glass forming process, specifically, at the end of the forming process, so that newly formed glass is protected substantially immediately after it is produced; among other things, to meet this criterion, the coating material should be (a) able to withstand the environment (e.g., high temperatures) of a glass forming line and (b) the method of applying the material should be safe for use in such an environment;
(2) the coating must protect the glass from chip adhesion resulting from cutting and/or grinding of the glass sheet, as well as the adhesion of other contaminants, e.g., particles, that the glass may come into contact with during storage and shipment prior to use;
(3) the coating must be sufficiently robust to continue to provide protection after being exposed to the substantial amounts of water which typically come into contact with the glass sheet during the cutting and/or grinding process;
(4) the coating preferably should protect the glass sheet from scratching during handling, shipping, and storage (as used herein, scratching includes abrasion);
(5) the coating must be substantially completely removable from the glass prior to its ultimate use in, for example, producing a liquid crystal display; and
(6) the coating should preserve the pristine glass surface without changing the surface""s chemistry or smoothness as a result of the coating being on the glass during handling, shipping, and storage.
The present invention addresses and satisfies this long standing need in the art.
The present invention provides methods for temporarily protecting a surface of a glass sheet from (1) ambient dirt, (2) chip adhesions, and (3) in its preferred embodiments, scratching.
In accordance with a first of its aspects, the invention provides a method for temporarily protecting glass from glass chips and/or scratching, said glass having at least one substantially flat surface, said method comprising:
(A) protecting the surface by:
(i) applying an aqueous solution comprising at least one polysaccharide (e.g., a starch) to the surface, said surface having a contact angle less than or equal to 8xc2x0 prior to the application of said aqueous solution; and
(ii) removing water from the aqueous solution to leave a polysaccharide-containing coating on the surface having a thickness of at least 0.01 microns (typically, the coated surface will have a contact angle of at least 25xc2x0, e.g., around 30xc2x0); and
(B) subsequently removing the polysaccharide-containing coating from surface using an aqueous solution;
wherein the surface has a contact angle after step (B) which is less than or equal to 8xc2x0.
In accordance with this aspect of the invention, the method can comprise the additional steps between steps (A) and (B) of:
(a) cutting the glass; and
(b) grinding and/or polishing at least one edge of the cut glass;
wherein:
water or a water-containing solution is applied to the coated surface during at least one of steps (a) and (b);
the surface of the coating has a first contact angle prior to steps (a) and (b) and a second contact angle after steps (a) and (b); and
the first and second contact angles differ by less than 5xc2x0.
In accordance with a second of its aspects, the invention provides an article of manufacture comprising:
(a) a glass sheet having at least one substantially flat surface; and
(b) a coating comprising at least one polysaccharide on the surface, said coating having a thickness of at least 0.01 microns;
wherein:
(i) the coating protects the surface from glass chips and/or scratching; and
(ii) the coating is removable from the surface through the application of an aqueous solution.
In certain preferred embodiments of the invention, the polysaccharide-containing coating is formed as part of the manufacturing process for the glass, wherein the manufacturing process produces newly formed glass at an elevated temperature and the newly formed glass is at a temperature above 150xc2x0 C. when it first comes into contact with the polysaccharide-containing aqueous solution. Although integration into a glass manufacturing process is preferred, the invention can also be practiced off-line if desired.
In other preferred embodiments, the coating is applied by spraying onto hot glass. Other approaches for applying the coating can be used, e.g., dipping, meniscus coaters, wick coaters, etc., but are less preferred when the invention is used as part of a glass manufacturing process since hot glass can often exhibit substantial back-and-forth movement at the end of such a process, especially, when a overflow downdraw process is used.
In further preferred embodiments, the coating is removed using an aqueous detergent solution, e.g., a commercial detergent package, preferably in combination with brush washing and/or ultrasonic cleaning. Typically, the aqueous detergent solution used to remove the coating will be heated to a temperature in the range from 40xc2x0 C. to 75xc2x0 C.
Other aspects of the invention are described in detail below.